This invention relates to a separator for an alkaline battery, in particular to a separator in an alkaline zinc-based battery, containing an insoluble inorganic salt, in particular a salt containing copper ions.
A zinc-based battery presents many challenges for the battery manufacturer. One of these challenges involves containing zincate diffusion within the battery. This is a particularly severe problem because zinc is extremely soluble in the strongly alkaline environments routinely used as the electrolyte in battery. The presence of KZn(OH)x where x=1 to 3 presents a double-edged sword for the battery designer. The high solubility allows for rapid current spikes typically unattainable with other battery systems. On the other hand, this high solubility permits diffusion of zinc ions to undesired locations within the battery, which upon re-plating leads to the well-known phenomenon of electrode shape change within zinc batteries. This shape change entails an agglommeration of the zinc towards the center of the battery with concomitant depletion from the edges.
Additionally, and more seriously, a problem arising from this high zinc mobility entails the accumulation of zinc dendrites within the separator as the battery cycles. Soluble zinc, driven by the electric field of the battery, finds its way through the pores of the separator. A concentration gradient within the separator leads, upon zinc re-plating, to dendrite formation. The dendrites are small tree-like metal formations that are capable of ripping the separator and prompting a cell short.
Prior art in this area has focused on diminishing dendrite formation by a variety of techniques. One technique involves physically diminishing the presence of zinc in critical areas where dendrite formation is not wanted. Thus, Colburn in U.S. Pat. No. 6,153,328 discloses a metal/air fuel cell that contains dendrite elimination zones characterized by the substantial lack of zinc in the volume of the zone and the substantial presence of a cell positive electrode through the volume.
Another approach involves adding complexing agents to the zinc electrode or adding agents to the electrolyte in order to suppress crystal growth. Lian et al in U.S. Pat. No. 5,830,601 teach an electrolyte active species comprising a metal hydroxide such as KOH or NaOH, and modifier such as a metal porphine, and/or a polyvinyl resin such as polyvinyl alcohol or polyvinyl acetate. Kawakami, et al in U.S. Pat. No. 5,824,434 use a multi-layered metal oxide film next to the zinc. U.S. Pat. No. 5,681,672 by Lee teaches adding barium fluoride to the electrolyte solution as a remedy for zinc dendrites. Adler et al in U.S. Pat. No. 5,302,475 use an electrolyte containing KOH and a combination of KF and K2CO3 salts, whereas Ando in U.S. Pat. No. 4,479,856 includes a quaternary ammonium salt and at least two metal ions selected from the group consisting of lead, tin and cadmium ions as dendrite inhibitor added to the electrolyte.
Another approach involves changing the electric field experienced by zinc. U.S. Pat. No. 5,780,186 by Casey, Jr. discloses specially treated zinc that fills the pores of a porous metal substrate. The use of indium sulphate as a coating agent for zinc is indicated in U.S. Pat. No. 5,626,988 by Daniel-Ivad, et al.
Yet another approach to prevent zinc dendrites has been to modify the separator. U.S. Pat. Nos. 4,154,912 and 4,272,470 feature crosslinks of polyvinyl alcohol by acetalization which supposedly forms networks between the molecules, thereby impeding zinc migration. In U.S. Pat. No. 6,033,806, Sugiura, et al discuss a similar crosslinked polyvinyl alcohol separator comprising additional steps involving an oxidizing agent to effect oxidative cleavage of 1,2-diol units and then acetalizing a film of crosslinked polyvinyl alcohol. Pemsler, et al in U.S. Pat. No. 4,592,973 disclose a separator with a hydrophobic microporous membrane whose pores are filled with a liquid ionic transport agent comprising an organic agent dissolved in an organic solvent. In U.S. Pat. No. 4,434,215, Wszolek, et al discloses a zinc-dendrite resistant separator made from copolymers of ethylene and acrylic or methacrylic acid. Shibley et al disclose in U.S. Pat. No. 4,371,596 a separator made from a porous, flexible substrate coated with an alkaline insoluble thermoplastic rubber-based resin and an alkaline reactive polar organic plasticizer along with polar particulate filler materials. A separator having the ingredients of a thermoplastic rubber, an inert filler and a processing agent selected from stearic acid, stearic acid metal salts, stearic acid amides, and natural or synthetic waxes is demonstrated in U.S. Pat. No. 4,327,167 by Feinberg et al.
The present invention provides a regenerated cellulose film resistant to zinc migration. This has been achieved by embedding an inorganic salt containing copper cations within the separator. The novel aspect of the present invention is that the copper salt impedes ready passage of zinc ions through the separator.
The battery separator of the present invention comprises cellulose, optionally containing hydrophobic agents, and an inorganic salt containing an insoluble copper(I) or copper(II) salt in a concentration range of 2 to 25% weight of said separator. This separator when placed in a zinc battery minimizes zinc dendrite formation and extends battery cycle life.
The novel aspect of our invention is to use a copper salt, embedded in a cellulose-based separator, as a dendrite inhibitor. Cellulose remains an excellent material as a separator for alkaline batteries because of its excellent ion transport properties and low electrical impedance. It has been used since World War II in the form of cellophane as the preferred separator material for zinc-based batteries. Cellulose as modified by the invention has enhanced ability to resist zinc dendrite penetration in an alkaline environment.